1. Field of the Invention
The present invention relates to mounting structures for noise filters, and more particularly, to a mounting structure for a noise filter including a four-terminal capacitor.
2. Description of the Related Art
FIGS. 6(A) and 6(B) illustrate a known noise filter relevant to the present invention (see, for example, Japanese Examined Utility Model Registration Application Publication No. 1-22258, Japanese Examined Utility Model Registration Application Publication No. 2-18587, and Japanese Examined Utility Model Registration Application Publication No. 6-31777). FIGS. 6(A) and 6(B) are a top view and a right side view, respectively, of a noise filter 1.
With reference to FIGS. 6(A) and 6(B), the noise filter 1 includes a four-terminal capacitor 2. The capacitor 2 has a chip-like shape and is provided with first and second terminal electrodes 5 and 6 on first and second end surfaces 3 and 4, respectively, which extend parallel to each other so as to face each other.
In addition, as shown clearly in FIG. 6(B), the noise filter 1 includes a hot side lead 10 that is bent in a U shape in such a manner that first and second leg portions 7 and 8 extend parallel to each other and a middle portion 9 connects the first and second leg portions 7 and 8 at one end thereof.
The hot side lead 10 is connected to the first terminal electrode 5 at the middle portion 9 thereof such that the first and second leg portions 7 and 8 extend parallel to the first and second end surfaces 3 and 4 of the capacitor 2. In addition, the first and second leg portions 7 and 8 of the hot side lead 10 respectively define a hot side input terminal connected to a power source and a hot side output terminal connected to a load circuit.
In the following description, reference numeral “7” is used to denote not only the “first leg portion” but also the “hot side input terminal”, and reference numeral “8” is used to denote not only the “second leg portion” but also the “hot side output terminal”.
The noise filter 1 further includes a ground side lead 14 that is bent in a U shape in such a manner that first and second leg portions 11 and 12 extend parallel to each other and a middle portion 13 connects the first and second leg portions 11 and 12 at one end thereof. The leg portions 11 and 12 of the ground side lead 14 are not completely shown in FIGS. 6(A) and 6(B).
The ground side lead 14 is connected to the second terminal electrode 6 at the middle portion 13 thereof such that the first and second leg portions 11 and 12 extend parallel to the first and second end surfaces 3 and 4 of the capacitor 2, that is, parallel to the first and second leg portions 7 and 8 in the hot side lead 10. In addition, the first and second leg portions 11 and 12 of the ground side lead 14 respectively serve as a ground side input terminal connected to a power source and a ground side output terminal connected to a load circuit.
In the following description, reference numeral “11” is used to denote not only the “first leg portion” but also the “ground side input terminal”. In addition, reference numeral “12” is used to denote not only the “second leg portion” but also the “ground side output terminal”.
A circuit board on which the above-described noise filter 1 is mounted includes a hot input electrode connected to the hot side input terminal 7, a hot side output electrode connected to the hot side output terminal 8, a ground side input electrode connected to the ground side input terminal 11, and a ground side output electrode connected to the ground side output terminal 12.
FIG. 7 is a schematic diagram illustrating an equivalent circuit of the noise filter 1 and the manner in which the hot side electrodes and the ground side electrodes of the circuit board are connected when the noise filter 1 is mounted on the circuit board. In FIG. 7, elements in the equivalent circuit of the noise filter 1 that correspond to the elements shown in FIGS. 6(A) and 6(B) are denoted by the same reference numerals in order to clarify the correspondence between the elements shown in FIG. 7 and those shown in FIGS. 6(A) and 6(B).
FIG. 7 shows a hot side input electrode 45 connected to the hot side input terminal 7, a hot side output electrode 46 connected to the hot side output terminal 8, a ground side input electrode 15 connected to the ground side input terminal 11, and a ground side output electrode 16 connected to the ground side output terminal 12. Although the overall body of the circuit board is not illustrated, the hot side input electrode 45, the hot side output electrode 46, the ground side input electrode 15, and the ground side output electrode 16 are provided on the circuit board.
The above-described circuit board is typically fixed to a chassis 17 made of metal with a metal screw 18. Accordingly, the ground side output electrode 16 is electrically connected to the chassis 17 via the metal screw 18.
When the noise filter 1 shown in FIGS. 6(A), 6(B) and 7 is in operation, a noise signal is transmitted from the ground side output terminal 12 to the chassis 17 via the ground side output electrode 16 on the circuit board and the metal screw 18. At this time, since the ground side output electrode 16 and the chassis 17 are only electrically connected to each other with the metal screw 18, there may be a case where the above-described noise signal cannot be sufficiently grounded.
In addition, in the circuit board, since the ground side input electrode 15 and the ground side output electrode 16 are disposed adjacent to each other and near each other, a stray capacitance 19 shown by the dashed lines in FIG. 7 is unavoidably formed.
Therefore, the noise signal that is transmitted from the ground side output terminal 12 to the ground side output electrode 16 and is grounded as described above is transmitted to the ground side input electrode 15 via the stray capacitance 19, as shown by the dashed arrows 44, and is then transmitted to the ground side input terminal 11, which reduces the noise reduction effect.
In addition, when the noise filter 1 shown in FIGS. 6(A), 6(B) and 7 is in operation, the noise signal is transmitted from the hot side output terminal 8 to the ground side output terminal 12 via the capacitor 2, as shown by the dashed arrows 47, and is then transmitted to the chassis 17 via the ground side output electrode 16 on the circuit board and metal screw 18. At this time, the impedance of the capacitor 2 at the noise frequency range may be high or the residual inductance of the ground side lead 14 may affect the noise signal as the residual impedance. In such cases, the above-described noise signal cannot be sufficiently grounded.
In the above-described situation, a portion of the noise signal transmitted from the hot side output terminal 8 to the capacitor 2 does not flow through the capacitor 2 but flows into the hot side lead 10 toward the hot side input terminal 7, as shown by the dashed arrows 48. In addition, a portion of the noise signal that flows through the capacitor 2 does not flow into the ground side lead 14 but flows toward the ground side input terminal 11, as shown by the dashed arrows 49. This also reduces the noise reduction effect.